Classification

There are ten classes
of molluscs; eight of the classes have living representatives, the
other two classes are known only from fossils. More than 250,000
species of mollusc are recognized and named. Snails (Gastropoda)
account for about 80% of living mollusc diversity.

Evolution

It is believed that the bivalves and scaphopods
are sister groups, as are the gastropods and cephalopods, as
indicated in the relationship diagram to the right.

In this phylum's level of organization, organ
systems from all three primary germ layers
can be found:

All major molluscan groups possess a skeleton,
though it has been lost through evolution in some members of the
phylum. It is probable that the pre-Cambrian
ancestor of the molluscs had calcium carbonate spicules embedded in
its mantle and outer tissues, as is the case in some modern
members.

The skeleton, if present, is primarily external
and composed of calcium carbonate (aragonite or calcite). The snail
shell or gastropod
shell is perhaps the best known molluscan shell, but many
pulmonate and opisthobranch snails have secondarily reduced and
internalized shells, or have lost the shell completely. The bivalve
or clam shell consists of two pieces (valves), articulated by
muscles and an elastic hinge. The cephalopod shell was ancestrally
external and chambered, as exemplified by the ammonoids and
nautiloids, and still possessed by Nautilus today. Other
cephalopods, such as cuttlefish, have internalized the shell, the
squid have mostly organic chitinous internal shells, and the
octopods have lost the shell altogether.

The first definitive evidence for molluscs comes
from an early Cambrian radula, but the Ediacaran organism Kimberella is
held by some to be an ancestral mollusc.

Dangerous molluscs

A very small minority of molluscs can
represent a serious risk to humans under the wrong
circumstances.

All octopuses are venomous but only a few species
pose a significant threat to humans, such as octopuses in the genus
Haplochlaena
which have a very poisonous bite. A few of the larger tropical
cone
snail species have a very poisonous sting. These bites and
stings can sometimes be fatal.

Some people are severely allergic to shellfish as
a food item. However, even for people without these allergies,
clams can sometimes be risky to eat. When there is a "red tide", or
other blooms of noxious plankton, or when there are
high concentrations of bacteria in the water from sewage run-off,
bivalves such as clams and mussels can temporarily become very
problematic as a food source. This is because bivalves are
filter-feeders, and thus they can concentrate toxins from floating
microorganisms within their tissues.

The traditional idea that the giant clam can
trap the leg of a person between its valves, thus drowning them,
has been shown to be a myth.